Squealing sound could be the sound of a new timing belt on an old tensioner. In my experience, the sound usually goes away after a few thousand miles after the new timing belt. I'm on my second timing belt, but the second time I had the tensioner replaced and didn't get the squealing like I did after the first. I also think my mechanic perhaps put the belt on a bit too tight.

Back to the engine racket, Mine happens just as you describe. Doesn't do it under harder acceleration (or at least I can't hear it... might just be shadowed by the sound of the engine itself) I mostly hear it when driving through parking lots or neighborhoods.. but I stopped paying any attention to it many thousands of miles ago.

 

It sounds like you have a problem with the valve timing. It's becoming inefficient for the air coming in and out when the valve is not at the correct timing. I suggest you have it adjusted at a bodyshop. It happened to my old 94 ex accord and 97 civic, luckily my friend is a mechanic and he fixed that for me, both cars.

Another case is the engine knocking, if you use lower octain gas than suggested in the manual, your engine will suffer a lot of knocking, and it will damage the engine over time. Just try higher octain gas and see what happens

 

Is it possible to buy 85 octane gas in this country